1. Field of the Invention
The present invention relates generally to removing liquid from wafers, and more particularly to apparatus and methods for drying a wafer that has been wet in a liquid bath, after which the wafer and the bath are separated at a controlled rate to form a thin layer of liquid on the wafer as the wafer is positioned in a gas-filled volume, wherein the volume is defined by a hot chamber that continuously transfers thermal energy to the wafer in the volume, and wherein hot gas directed into the volume and across the wafer and out of the volume continuously transfers thermal energy to the wafer, so that the thermal energy transferred to the wafer in the volume evaporates the thin layer from the wafer without decreasing the rate of separation of the wafer and the bath below a maximum rate of such separation at which a meniscus will form between the bath and the surface of the wafer during such separation.
2. Description of the Related Art
In the manufacture of semiconductor devices, process chambers are interfaced to permit transfer of wafers between the interfaced chambers. Such wafer transfer is via transport modules that move the wafers, for example, through slots or ports that are provided in the adjacent walls of the interfaced chambers. For example, transport modules are generally used in conjunction with a variety of wafer processing modules, which may include semiconductor etching systems, material deposition systems, flat panel display etching systems, and wafer cleaning systems. Due to growing demands for cleanliness and high processing precision, there has been a greater need to reduce the amount of human interaction during, between, and after such processing steps. This need has been partially met with the implementation of vacuum transport modules which operate as an intermediate wafer handling apparatus (typically maintained at a reduced pressure, e.g., vacuum conditions). By way of example, a vacuum transport module may be physically located between one or more clean room storage facilities where wafers are stored, and multiple wafer processing modules where the wafers are actually processed, e.g., etched or have deposition performed thereon, or cleaned. In this manner, when a wafer is required for processing, a robot arm located within the transport module may be employed to retrieve a selected wafer from storage and place it into one of the multiple processing modules.
Despite use of such intermediate wafer handling apparatus, it is still necessary to clean and dry the wafers at the completion of such processing. As an example, after the wafers have been cleaned, the wafers may have a non-uniform coating of liquid. A wafer with such non-uniform coating of liquid, or with one or more drops of liquid thereon, or with any liquid thereon in any physical form, may be said to be xe2x80x9cwetxe2x80x9d. In contrast, a wafer having a uniform coating of liquid may be said to be xe2x80x9cuniformly wetxe2x80x9d.
In the past, annular-shaped pieceparts other than wafers have been subjected to a drying operation. After cleaning and while wet, such pieceparts have been placed in a tank containing a bath of hot liquid. In one type of drying operation, the hot liquid has been drained from the tank at a rate such that a thin layer of liquid, rather than one or more drops of liquid, forms on that portion of such piecepart that is out of the draining liquid. The thin layer has been preferred over one or more drops because a drop of liquid has a high volume, e.g., from about 0.001 ml. to about 0.020 ml. In comparison to the drop, a thin layer of liquid on a wafer such as a 200 mm. diameter wafer, may only have a volume at the maximum diameter of about 0.133 of 0.0105 ml., which is the middle of the above volume range of the drop, for example. Evaporation of a drop generally results in the concentration of small particles at the last small point at which the drop exists. When the piecepart is a wafer, such concentration may result in defects in a chip made from the wafer.
To remove the thin layer from such piecepart, reliance has been placed on the thermal energy stored in such piecepart to provide the thermal energy necessary to evaporate the thin layer. However, when such pieceparts are xe2x80x9cwafersxe2x80x9d, as defined above, problems have been experienced in not properly drying the thin layer from the wafer. For example, it appears that using only such stored thermal energy, the thin layer evaporates from the wafer at a rate less than the maximum rate of separation of the liquid bath and the wafer at which a meniscus will form between the liquid bath and the surface of the wafer during such separation. Thus, the rate at which the liquid is drained from the tank has to be decreased to match the rate of evaporation. Alternatively, the wafer would have to be retained in the tank after the draining has been completed. Each of such decreased rate of draining and such retaining increases the time required to dry the wafer, which increases the cost of fabricating devices based on the wafer.
In view of the forgoing, what is needed is apparatus and methods of efficiently drying wafers. Such efficient drying should allow the wafers and the liquid to be separated at a rate no less than the maximum rate of separation of the liquid and the wafer at which a meniscus will form between the liquid bath and the surface of the wafer. Also, the efficient drying should rapidly remove from the wafer a thin layer of liquid that forms on the wafer as the wafer and the bath are separated, wherein xe2x80x9crapidlyxe2x80x9d means such removal occurs before the wafer and the bath have been completely separated e.g., separated by about 0.004 inches.
Broadly speaking, the present invention fills these needs by providing apparatus and methods of efficiently removing fluid from wafers. The efficient removing is attained by providing apparatus and methods for drying a wafer that has been uniformly wet in a fluid bath, in which the wafer and the bath are separated at a controlled rate to form a thin layer of fluid on the wafer as the wafer is positioned in a gas-filled volume. In addition to such separation, the efficient removing is attained by defining the gas-filled volume by use of a hot chamber that continuously transfers thermal energy to the wafer in the volume. Further, hot gas directed into the volume and across the wafer and out of the volume continuously transfers thermal energy to the wafer. The directing of the gas out of the volume is independent of the separation of the bath and the wafer. The thermal energy transferred to the wafer in the volume evaporates the thin layer from the wafer without decreasing the rate of separation of the wafer and the bath below the maximum rate of such separation at which a meniscus will form between the bath and the surface of the wafer during such separation. In addition to such separation and directing of the hot gas across the wafer and out of the volume, the relative humidity in the volume is kept low to inhibit recondensation of the fluid on the wafers, for example.
Such efficient removal enables the wafer throughput of such apparatus and method to be limited only by the type of wafer that is being dried, and the type of fluid used to wet the wafer. For example, the characteristics of particular types of wafers and fluid dictate the maximum rate of such separation of the wafer and the bath at which a meniscus will form between the bath and the surface of the wafer during such separation and the wafer will be uniformly wet.
In one embodiment of the present invention a wafer drying system may include a bath enclosure configured to hold a fluid so that the fluid defines a top fluid surface. A temperature and humidity-controlled chamber may also be defined above the fluid surface. The chamber has a first opening at a first side proximate to the fluid surface and a second opening at a second side that is opposite to the first side.
In another embodiment of the present invention the wafers to be dried have opposite sides, and apparatus for drying the wafers may include a bath containing hot liquid, wherein the liquid defines an upper surface. Also provided is an enclosure having an inlet spaced from the upper surface and an outlet adjacent to the upper surface. The enclosure defines a continuous gas flow path from the inlet to the outlet, the flow path extending from the inlet along the upper surface and through the outlet. A heat transfer unit may supply hot gas to the inlet, with the hot gas being under pressure so as to flow in the continuous flow path. The heat transfer unit may transfer thermal energy to the enclosure so that the enclosure radiates thermal energy across the continuous flow path. A wafer carrier may be movable in the bath and in the enclosure for moving the wafer at a controlled rate out of the bath and into intersection with the continuous flow path. The rate may be controlled so that as the wafer moves out of the bath a thin layer of the liquid is formed on each of the opposite sides of the wafer. As the wafer intersects the continuous flow path thermal energy from the hot gas and from the enclosure is received by the wafer and by the thin layer. The received thermal energy evaporates the thin layer off the opposite sides of the wafer.
In a related embodiment, the walls of the enclosure may define a perimeter of the enclosure. A plenum surrounds the perimeter of the enclosure for receiving the gas and the evaporated thin layer from the outlet. To assure that the flow path remains continuous and to control the relative humidity in the enclosure, a fan is provided for exhausting the gas, the evaporated thin layer, and vapor from the bath from the plenum. In a further embodiment, apparatus provided for drying a wafer having opposite planar sides may include a bath for containing a fluid having an upper surface. A heat transfer chamber may have a plurality of walls, each of the walls having a bottom at generally the same level as the level of adjacent ones of the walls. The chamber defines a wafer drying volume above the bottoms of the walls and within which a wafer drying path extends. At least one of the walls is provided with a gas inlet positioned opposite to the bottom. A support may suspend the chamber above the bath with the wafer drying path starting adjacent to the fluid surface and extending to a point adjacent to the gas inlet. The support positions the bottoms of the chamber walls spaced from the liquid surface to define an elongated outlet extending around the wafer drying path. A hot gas supply may be connected to the gas inlet for flowing hot gas through the chamber across the opposite planar sides of the wafer and out of the chamber through the elongated outlet to continuously transfer thermal energy at a selected temperature across the wafer drying path, and thus to the wafer and the thin film on the wafer. A heater connected to the chamber between the gas inlet and the elongated outlet may radiate thermal energy across the wafer drying path, and also to the wafer and the thin film on the wafer.
In a still other embodiment, a method for drying a wafer may include an operation of introducing a wafer being in a wet state into a fluid bath. The wafer is removed from the fluid bath at a controlled rate along a selected path. Heated gas is applied to the wafer as the wafer is moved along the selected path and out of the fluid bath. Advantageously, the applied heated gas flows in at least one continuous flow path to the wafer without recirculating the heated gas to the wafer. In this manner, the applied heated gas transitions the wafer to a dry state as the wafer exits the fluid bath. A related feature is that thermal energy is radiated onto the wafer as the wafer moves along the selected path out of the fluid bath. In another related aspect of this method embodiment, an enclosure is provided to define the at least one continuous flow path. The applying of the heated gas may include flowing hot nitrogen in the at least one continuous flow path across the wafer to effect the transition by evaporating the fluid from the wafer into the hot nitrogen. The applying operation then removes the hot nitrogen and the evaporated fluid from the enclosure and away from the fluid bath. In this manner, the hot nitrogen and the evaporated fluid are not recirculated in the enclosure, such that the evaporated fluid does not accumulate, which accumulation would reduce the rate at which the evaporation takes place and foster recondensation of the fluid on the wafers.
In yet another embodiment of the present invention a method for drying a wafer may cause a wafer to be immersed in a fluid bath to wet opposite sides of the wafer with the fluid. Then the wafer is moved out of the fluid bath into a defined volume along a selected path. The moving may be controlled to allow a meniscus on each of the opposite sides to form and leave a thin film of the fluid on the opposite sides of the wafer as the wafer moves from the fluid bath. By directing radiant energy into the thin film of the fluid on the opposite sides of the wafer, and by flowing heated gas into the defined volume and along the wafer as the wafer is moved along the selected path out of the fluid bath, the thin film of the fluid is evaporated from the wafer and combines with the heated gas flowing along the wafer. An exit from the defined volume is provided for the combined removed thin film of the fluid and the gas. Advantageously, the combination of the radiant energy, the heated gas and the exit promote rapid evaporation of the thin film and foster a decrease in the time required to dry the wafers.
Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.